Furnaces

Oxide film growth is mainly a batch process. Up to 150 wafers are loaded into a furnace, where they are simultaneously oxidized. Since the process is temperature-dependent, this parameter is tightly controlled over the oxidation tube's length. This is why furnaces are crucial in IC manufacturing. They are of three types: conventional horizontal and vertical furnaces, and rapid thermal processors (RTPs).

Until the mid-1980s, hot-wall horizontal diffusion furnaces were the industry workhorses. Vertical furnaces began replacing them, and now RTPs are displacing vertical furnaces in some applications. Fast-ramp, small-batch vertical furnaces now compete with RTP systems in processes where thermal budget and throughput are important.

A horizontal furnace consists of a furnace cabinet, heating elements, thermocouples for measurement and control, fused-silica process tubes, paddles and boats, temperature control system, load station, source gas cabinet and gas delivery system.

Furnaces enable oxide film growth. As the industry has evolved, furnace technology has kept pace by changing its features, but always with the same goal: to lay down the best film possible. (Source: Stanley Wolf, Microchip Manufacturing, reprinted with permission of Lattice Press)

Wet oxidation is typically used to grow thicker oxides, because the growth rate is faster than for dry oxidation. The H₂O species needed for the reaction is obtained from a bubbler or an H₂ and O₂ pyrogenic reaction. In the pyrogenic method, hydrogen and oxygen are separately fed into the heated furnace tube. Hydrogen ignites spontaneously in the presence of oxygen at about 400°C, forming steam.

Despite their higher cost, vertical furnaces are preferred over horizontal in many applications because they offer superior process control, less contamination, and better automation compatibility. Temperature within the furnace is more uniform, and the wafers are held flat and well centered in the boat. This optimizes gas flow dynamics and the boat can be rotated, averaging out temperature and gas variations.

In a vertical furnace, the double-walled fused-silica process tube allows inert or chlorinated gases to be flowed between the inner and outer tubes, preventing impurities from diffusing through the inner tube and entering the process region. A fused-silica boat horizontally holds up to a 150-wafer batch. Robots load the wafers onto the boat from a cassette. The boat is moved into the furnace's hot zone, which is held between process steps at lower temperature (700-800°C). Then the temperature is slowly raised to the process value (~950°C). Afterward, the temperature is slowly reduced until the wafers are cool enough to be withdrawn.

Recently, a new vertical furnace appeared: the fast-ramp small-batch furnace. It rapidly raises the temperature of a 50-wafer batch to the process temperature, and quickly cools them after completion. The ramp-up rate is increased to 100°C/min from l0-20°C/min in conventional furnaces, and ramp-down rate is 60°C/min (compared with 5°C/min). This makes it possible to process a smaller batch of wafers with a much shorter overall cycle time.

RTP is a single-wafer processing method in which process temperature is rapidly ramped up and down at a rate of 75-200°C/sec, compared with <1°C/sec in a furnace. RTP can heat a wafer from room temperature to 1100°C in seconds. It offers advantages over furnaces, including a reduced thermal budget, higher-temperature processing, better process ambient control, shorter process time, and ease of clustering multiple tools. It is preferred for applications such as post-ion implantation annealing and activation, titanium- and cobalt-silicide annealing and activation, and rapid thermal oxidation to form ultrathin gate oxide layers.

However, furnaces still perform well enough for some applications, and are less expensive. RTP will be increasingly required for production processes used to make 0.18 µm feature sizes on 300 mm wafers.

The goal of thermal oxidation is to grow a defect-free, uniform SiO₂ layer to a specified thickness. Thin oxides are commonly grown using dry oxidation. In a typical growth sequence for producing oxides ~50-100 Å thick in a batch-oxidation furnace, the wafers are first cleaned. While the furnace is idling, nitrogen purge gas is flowed through it, and it sits at an elevated temperature (~800°C). The wafer boat is slowly moved into the chamber — with N₂ gas flowing — to avoid warping caused by temperature changes.

When the wafers are inside, furnace temperature is increased at a rate of 10-20°C/min. After the process temperature is reached (e.g., 950°C), the furnace is given a few minutes to stabilize under N₂ flow. Then dry oxygen (and perhaps a gas containing chlorine) is turned on, and the nitrogen is stopped. While the oxygen flows, the oxide grows. At the specified thickness, the gases are turned off.

The wafers then undergo a post-oxidation anneal for about 30 min in nitrogen at process temperature. Next, the furnace temperature is slowly ramped down, and when the idle temperature is reached, the boat is slowly withdrawn.

This material has been adapted from Microchip Manufacturing, by Stanley Wolf.